Generally, in outboard engine units, a drive shaft for transmitting rotation of an engine extends vertically, a pair of bevel gears meshingly engage with a pinion (i.e., pinion bevel gear) provided on a lower end portion of the vertical drive shaft, and a propeller shaft is connected to the bevel gears via a clutch (dog clutch). The pinion, the pair of bevel gears and the clutch are accommodated in a gear chamber within a gear case, and the gear chamber is filled with lubricant oil.
A thrusting propeller is mounted on the propeller shaft, and a shift rod is connected to the clutch. The shift rod is accommodated in a shift chamber within the gear case. By the clutch being operated via the shift rod, switching can be made among a forward rotation state in which one of the pair of bevel gears is connected to the propeller shaft, a reverse rotation state in which the other of the pair of bevel gears is connected to the propeller shaft and a neutral state in which the pair of bevel gears are disconnected from the propeller shaft.
Further, the drive shaft is rotatably supported at its support shaft section, located above the pinion, in the gear case by means of a taper roller bearing. The taper roller bearing is provided above the oil level of the lubricant oil filled in the gear case. Thus, a lubrication device is provided for directing the lubricant oil upward to the taper roller bearing. To that end, the lubrication device includes an oil slinger located underneath the taper roller bearing of the drive shaft and communicating with the gear chamber via a supply passage, and the taper roller and the shift chamber are in communication with each other via a return passage. A helical guide groove is formed in the outer periphery of the oil slinger.
One example of such outboard engine units is disclosed in JP-A 2004-262397 (hereinafter referred to as “the relevant patent literature”), according to which rotation of the drive shaft is transmitted to the bevel gears to rotate the bevel gears, so that the lubricant oil of the gear chamber is directed to the oil slinger via the supply passage. The oil slinger rotates together with the drive shaft, and by such rotation of the oil slinger, the lubricant oil directed to the oil slinger is then directed, via the helical guide groove, to the taper roller bearing, so that the taper roller bearing is lubricated with the lubricant oil. The lubricant oil having lubricated the taper roller bearing is then directed to the shift chamber via the return passage. The lubricant oil having been directed to the shift chamber is returned from the shift chamber back to the gear chamber. By circulating the lubricant oil in the aforementioned manner, the taper roller bearing can be lubricated with the lubricant oil.
In order to enhance durability of a power transmission system of the outboard engine unit, particularly the pinion and the pair of bevel gears (hereinafter collectively referred to as “gear mechanism”), increasing the size of the gear mechanism may be a conceivable option. Further, in the outboard engine unit disclosed in the relevant patent literature, the gear mechanism is accommodated in the gear chamber, and thus, if the size of the gear mechanism is increased, an inner space of the gear chamber would be decreased. Hence, as the lubricant oil is circulated to lubricate the taper roller bearing, inner pressure of the gear chamber would increase so that it becomes difficult to retain the lubricant oil within the gear chamber by means of a sealing member.
As a countermeasure against such an inconvenience, it is conceivable to increase the size of the gear case of the outboard engine unit to thereby increase the inner space of the gear case. However, if the size of the gear case is increased like this, resistance of seawater would increase during sliding travel of the hull or body of the watercraft, which would result in a cause of lowered capability (sliding travel capability) of the outboard engine unit.